JPS6280545A - Environmental abnormality detecting circuit - Google Patents

Abstract

PURPOSE:To obtain a circuit which is hardly affected by noises, temperature drift, etc., by resetting a presettable counter which counts the output of a meas uring oscillation circuit varying in frequency with the resistance value of a sensor at a constant period. CONSTITUTION:The output of the measuring oscillation circuit 2 is applied to the counter input terminal A of the presettable counter 1. This circuit 2 consists of the sensor S which varies in resistance value according to environ mental states such as temperature, flammable gas concentration, etc., a resis tance R1, a capacitor C0, and CMOS inverters IC1 and IC2, and is set to a specific frequency previously. Further, the output of a reference oscillation circuit 3 is applied to the reset terminal C of the counter 1 through a capacitor C2 and a resistance R3 constituting a differentiating circuit and the counter is reset on the positive pulse side at the constant period. This circuit 3 is so constituted that the sensor S of the circuit 2 is replaced with a fixed resistance R2. Thus, digital processing is carried out without using any DC amplifier, so the circuit is hardly affected by noises, temperature drift, etc.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] This invention relates to improvements in detection circuits for detecting environmental parameters such as temperature and gas concentration.

[Conventional technology]

Conventionally, a detection circuit that detects the temperature inside a building or gas concentration as a change in resistance value, such as a thermistor or gas detection element, is as shown in Figure 2.
A bridge circuit is constructed from a series circuit of sensor S and a resistor, and resistors R2 and R8, and the output of this bridge circuit is detected through a differential amplifier 1 and a threshold circuit 2, respectively. be.

[Problem that the invention seeks to solve]

However, conventional detection circuits such as those described above may malfunction due to single pulses such as noise.
The set temperature changes due to a slight drift of the amplifier, and therefore has drawbacks such as requiring an expensive amplifier and difficulty in changing the temperature setting.

[Failure to solve the problem]

The environmental abnormality detection circuit of the present invention includes a sensor that detects changes in physical phenomena in the surrounding environment such as temperature and gas concentration, a measurement oscillation circuit whose frequency changes according to changes in the resistance value of the sensor, and the measurement oscillation circuit. The above-mentioned problem has been solved by comprising a presettable counter that counts the output of , and a reference oscillation circuit that resets the presettable counter at regular intervals.

〔Example〕

FIG. 1 shows an embodiment of the present invention, in which the output of a measurement oscillation circuit 2 is applied to a counter input terminal A of a presettable counter 1. This measurement oscillation circuit 1 is composed of a sensor S whose resistance value changes depending on environmental conditions such as temperature and combustible gas concentration, a resistor R1, a capacitor C, a CMOS inverter IC, and an IC2. frequency is set.

Further, the output of the reference oscillation circuit 3 is applied to the reset terminal C of the presettable counter 1 via a capacitor C2 and a resistor R8 that constitute a differentiating circuit. The circuit 2 is composed of the same components as the measurement oscillation circuit 2, except that a fixed resistor R2 is connected to the sensor S connection portion of the circuit 2, and the value of this resistance is different from that of the capacitor C1.

Furthermore, the preset terminal of presettable counter 1,
, D2 . Threshold levels can be set for stages.

FIG. 3 shows output waveforms from various parts of the circuit to explain the operation of the circuit shown in FIG. As shown in this figure, the oscillation frequency of the measurement oscillation circuit 2 during normal operation is set higher than the oscillation frequency of the reference oscillation circuit 3.

The operation when the detection circuit configured as described above is used as, for example, a fire detector will be explained.

First, an n-pit counter is used as the presettable counter 1, and a count number N is set in advance. Further, a thermistor is used as the fire detection sensor S, and is connected to the measurement oscillation circuit 2. Oscillation period T3 of each oscillation circuit 2, 3 at this time
．． The relationship between T4, the resistance value R3 of the sensor S, the resistance R2, and the capacitor C6+''I is as follows.

T2-Guo・R8-CoT, -Chopsticks・R2・C1i...Constant T2(2°-N)-T3 Z"-1N shi.

Note that these equations do not include functions of the power supply voltage or ambient temperature, and it can be seen that they are not affected by these.

As the temperature rises over time t due to the occurrence of a fire, the resistance value R3 of the thermistor S gradually decreases, and the oscillation period T of the measurement oscillation circuit 2 increases.
2 changes as shown in FIG. 3(A).

Further, the output of the reference oscillation circuit 3 is shown in FIG. 3(B).
This output becomes a waveform as shown in FIG. 3(C) by a differentiating circuit consisting of a capacitor C2 and a resistor R8, and is applied to the reset terminal C of the counter 1, and is reset on the positive pulse side.

When the ambient temperature is normal, presettable counter 1 is
Although it is reset before reaching the preset count number N, when a predetermined temperature is reached, the output terminal of the presettable counter 1 produces an output as shown in FIG. 3(D). This signal may be applied to an alarm circuit (not shown) to generate a fire alarm.

The threshold temperature may be adjusted using the preset switch 4.

Further, in this embodiment, the resistance R of the reference oscillation circuit 3
A fixed resistor was used as R2, so it was used as a so-called fixed temperature fire detector. However, if a heat-sensitive element is used as the resistor R2, and it is installed so that it is difficult to come into contact with the outside air and slowly follows changes in the outside air temperature, it will prevent a fire. The detector can also be operated as a so-called differential fire detector.

Figure 2 shows another embodiment of the measurement oscillation circuit 2, in which a first variable resistor R4 is provided in series with the sensor S, and a second variable resistor R4 is inserted in parallel with the sensor S. When adjusting the threshold value, use these variable resistors R4 and R
, can be done. The resistance value, capacitor value, etc. at this time have the following relationship.

By using a variable resistor in this way, a much simpler presettable counter 1 can be used.

〔Effect of the invention〕

The environmental abnormality detection circuit of the present invention uses digital processing without using a DC amplifier as described above, so it is less susceptible to noise and has a threshold value that is not affected by fluctuations in power supply voltage or temperature drift. Does not affect the level. Furthermore, the threshold value can be set finely and easily.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing one embodiment of the present invention, FIG. 2 is a diagram showing a conventional example, FIG. 3 is a time chart for explanation of FIG. 1, and FIG. Figures showing main parts of the embodiment are shown. ■...Presettable counter, 2...Measurement oscillation circuit, 3...Reference oscillation circuit, 4...Preset switch, S...Sensor.

Claims (1)

[Claims] A sensor that detects changes in physical phenomena in the surrounding environment such as temperature and gas concentration, a measurement oscillation circuit whose frequency changes depending on changes in the resistance value of the sensor, and a presettable counter that counts the output of the measurement oscillation circuit. , and a reference oscillation circuit that resets the presettable counter at regular intervals.